A cylinder assembly for a two-stroke engine is known from U.S. Pat. No. 5,947,066 wherein the lateral transfer channels are configured in the cylinder wall as radial channels opening outwardly. The transfer channel of each side is closed radially to the outside by a cover. In this way, curved channels are formed which run curved to the cylindrical axis. If more than two transfer channels are on one side of a cylinder, then the manufacture in die cast is no longer possible because of the different angles and channel paths.
It is an object of the invention to provide a die cast metal cylinder assembly for a two-stroke engine which makes two and more transfer channels possible on opposite-lying sides. The cylinder assembly of the invention is for a two-stroke engine including an engine for a portable handheld work apparatus including a chain saw, brushcutter, cutoff machine or the like. The two-stroke engine includes a piston. The cylinder assembly includes: a cylinder having a cylinder wall defining a cylinder bore surface and a cylinder axis; the cylinder wall and the piston conjointly delimiting a combustion chamber in the engine; an outlet formed in the cylinder wall for conducting combustion gases away from the combustion chamber; a plurality of transfer channels for conducting operating means to the combustion chamber for operating the engine; a first one of the transfer channels having a first transfer window opening into the combustion chamber near the outlet to form an outlet-near transfer window; a second one of the transfer channels having a second transfer window opening into the combustion chamber remote from the outlet to form an outlet-remote transfer window; the first and second transfer channels being open channels directed radially outwardly; a cover mounted on the cylinder to cover the channels; the first and second windows having respective outlet-remote frame surfaces extending approximately parallel to the cylinder axis and facing toward the outlet viewed in the peripheral direction of the cylinder bore surface; and, the outlet-remote frame surfaces lying in respective mutually parallel pull planes.
The alignment of the transfer channels and the transfer channel windows to the combustion chamber is such that the inflow direction of the inflowing gases is different. The different inflow direction causes different angles of the channel side walls. According to the invention, the frame surfaces of the outlet-near and outlet-remote transfer windows on one side are arranged in pull planes parallel to each other. The frame surfaces run approximately parallel to the cylinder axis and lie facing toward the outlet in the peripheral direction of the cylinder bore. For this reason, the possibility is provided to form the two transfer windows, which lie on one side, with core sliders movable radially from the outside without backcuts, projections or the like. The core sliders can be moved individually or together in mutually parallel pull planes. The axial opening of the transfer channels to the crankcase is provided by axial core sliders which are to be moved in the direction of the cylinder axis.
The core sliders forming the transfer windows are so configured that they simultaneously form the outer frame surfaces of the transfer windows together with the external channel side walls.
In order to configure the different inflow directions, it is provided that the frame surfaces of a transfer window, which run approximately in the longitudinal direction of the cylinder axis, lie in planes which intersect in the combustion chamber. The intersect angle of the planes of the first outlet-near transfer channel window is greater, preferably by 0.2 to 0.5 times greater, than the intersect angle of the planes of the second outlet-remote transfer window.
The frame surface of the first, outlet-near transfer window lies facing toward the outlet and extends with a step to the channel center wall configured in the cover. In this way, the edge of the frame surface (which lies facing toward the outlet) of the first transfer window is so arranged that it lies on the inner surface of the cylinder bore as well as in the plane of a side surface (formed by the channel center wall) of the outlet-near transfer channel. This edge of the frame surface of the first transfer window runs along the cylinder axis and faces toward the combustion chamber.
A recess, notch or the like in the material of the cylinder is advantageously provided between the outlet-near transfer channel and the outlet. This is provided in order to avoid an increased thermal transfer from the outlet to the outlet-near transfer channel.